1. Field of the Invention
The present invention relates in general to a servo system which constitutes a tape travelling system in a video cassette recorder (VCR) deck mechanism, and more particularly to a circuit and a method for controlling a speed and a phase of the servo system constantly using a capstan frequency generation (CFG) pulse in a blank interval of a control (CTL) pulse when a video tape is winding.
2. Description of the Prior Art
Referring to FIG. 1, there is schematically shown a general tape travelling system in a VHS type of VCR deck mechanism.
During operation, a video tape 2 supplied from a supply reel 1 is passed in a recording mode through a full erase head 3, with information previously recorded therein being erased. Then, the video tape 2 is passed through a drum 4, with a video signal being recorded or played back thereon/therefrom.
A control (CTL) pulse is recorded or reproduced on/from a control track of the video tape 2 by a recording/playback control head 5. Finally, the video tape 2, through the control head 5, is pressed between a pinch roller 6 and a capstan shaft 7 and then wound about a take-up reel 8 by the pressure.
For the purpose of transferring the video tape in the VHS type of VCR servo system, a speed and a phase of the servo system must be controlled. The speed control is performed based on an amount of rotations of a capstan motor 9 and the phase control is performed based on the CTL pulse reproduced from the control track of the video tape.
The capstan shaft 7 must be controlled constantly by the capstan motor 9 to ensure that the video tape is stably wound. The constant speed travelling of the video tape is directly connected to a picture quality in recording and playing back of the video signal, and results in an accurate determination of the remaining amount of the video tape required in reserve-recording the video signal. The remaining amount of the video tape is basic information in controlling a tape winding speed in FF/REW modes. For this reason, the constant speed travelling of the video tape must be controlled accurately.
A circuit for discriminating the remaining amount of the video tape comprises reflection plates 10 and 11 mounted to the supply reel 1 and the take-up reel 8, respectively. The reflection plates 10 and 11 rotate with the supply reel 1 and the take-up reel 8, respectively, and each has a plurality of bright and dark portions (for example, 8 or 16 equal parts) partitioned in a fixed angle. Also in the circuit, photocouplers 12 and 13 are provided to generate pulses corresponding to reflected light regulated by the reflection plates 10 and 11, respectively, in the during the winding of the video tape. Wave-shapers 14 and 15 are also provided to wave-shape the pulses from the photocouplers 12 and 13, respectively.
In a method of discriminating the remaining amount of the video tape with the above-mentioned construction, the reflection plates 10 and 11 rotate as the supply and take-up reels 1 and 8 rotate while the video tape is winding. As the reflection plates 10 and 11 rotate, the reflected light is regulated by the bright and dark portions of the reflection plates 10 and 11. Then, the photocouplers 12 and 13 generate the pulses corresponding to the reflected light regulated by the reflection plates 10 and 11, respectively. The wave-shapers 14 and 15 wave-shape the pulses from the photocouplers 12 and 13, respectively, and apply the wave-shaped pulses to an operation circuit (not shown). The operation circuit is adapted to measure a period of the rotation of each reel by counting the pulses and discriminate the type of the video tape and obtain the remaining amount thereof by calculating an angular velocity of each reel, a diameter of each hub and an area of the video tape wound about each reel based on the measured rotation period.
In calculating the remaining amount of the video tape, linear velocity, integral calculus and angular velocity are determined. The type of the video tape includes, for example, T-20, T-40, T-60, T-80, T-120, T-160 or T-180. The video tapes of the T-20 type to T-60 type have the same thickness and in this case the sizes of the hubs are the same. The video tapes of the T-80 type and T-120 type have the same thickness as that of the T-20 type, however in this case, the hubs have the same size as those of the hubs in the video tapes of the T-160 type and the T-180 type, respectively. Also, the video tapes of the T-160 type and the T-180 type have different thicknesses. Since the video tapes of various types have different thicknesses and the sizes of the hubs are different for the tapes mentioned above, there is a necessity for discriminating the type of the video tape being presently used and calculating the remaining amount thereof based on an equation.
As mentioned above, the period of the rotation of the reel is used in discriminating the type of the video tape and calculating the remaining amount thereof, and an accuracy in the measurement of the rotation period plays an important part in the calculation of the remaining amount of the video tape. That is, the reserve-recording of the video signal may be performed in a treble speed mode in the case where the amount of the video tape is not enough when the video signal is reserve-recorded on the video tape at a standard speed. Also, in the FF/REW modes, the middle portion of the video tape is wound at the highest speed allowable by the deck mechanism and the initial and end portions thereof are wound at a lower speed, resulting in prevention of the deck mechanism and the video tape from being damaged at the initial and end portions of the video tape.
Referring to FIG. 2, there is shown a schematic block diagram of a conventional circuit for controlling the speed and the phase of the VCR servo system using the control pulse. As shown in this drawing, the conventional control circuit comprises a CTL signal generation section 70 for generating the control signal to be recorded on the control track of the video tape, upon inputting a recording mode signal S1, a speed error detection section 20 for detecting a capstan frequency generation (CFG) signal from the capstan motor 9 during the travelling of the video tape to detect a speed difference between the CFG signal and a reference speed signal Sref from a speedometer and outputting the detected speed difference as speed control data, and a phase error detection section 30 having a recording phase error detector 30A and a playback phase error detector 30B.
The recording phase error detector 30A is adapted to detect a recording phase error from a CFG pulse from the capstan motor 9 in a recording mode and output the detected recording phase error as phase control data. The playback phase error detector 30B is adapted to detect a playback phase error from the CTL pulse recorded on the control track of the video tape in a playback mode and output the detected playback phase error as phase control data.
The conventional control circuit also comprises a speed/phase control section 40 and first and second switching sections 50 and 60.
The speed/phase control section 40 is adapted to input the speed control data from the speed error detection section 20 and the phase control data from the phase error detection section the speed/phase control section 40 controls the speed and phase of the capstan motor 9 in accordance with the inputted data.
The first switching section 50 is operative in response to the recording mode signal S1 to select the output from the playback phase error detector 30B in the phase error detection section 30 in the playback mode and select the output from the recording phase error detector 30A in the recording mode.
The second switching section 60 is operative in response to an FF/REW mode signal S2 to apply no phase control data selected by the first switching section 50 to the speed/phase control section 40 in the FF/REW modes and apply the phase control data selected by the first switching section 50 to the speed/phase control section 40 in modes other than the FF/REW modes.
Referring to FIG. 3, there is shown a detailed block diagram of the conventional control circuit in FIG. 2. As shown in this figure, the speed error detection section 20 includes a CFG detector 21 for successively detecting the CFG signals from the capstan motor 9, a wave-shaper 22 for wave-shaping the detected CFG signals from the CFG detector 21 into the CFG pulses in the form of a square wave, a period measurer 23 for sequentially measuring a period of each CFG pulse from the wave-shaper 22, a converter 24 for converting the period value from the period measurer 23 into a time value, a subtracter 25 for subtracting the reference speed value Sref from the speedometer from an output signal from the converter 24 and outputting the resultant difference as a speed error signal, and a multiplier 26 for multiplying the speed error signal from the subtracter 25 by a predetermined integral number from the speedometer and outputting the resultant value as the speed control data.
As mentioned above, the phase error detection section 30 is provided with the recording phase error detector 30A for detecting the phase error of the video tape in the recording mode and the playback phase error detector 30B for detecting the phase error of the video tape in the playback mode.
The recording phase error detector 30A includes a frequency-divider 31 for frequency-dividing the CFG pulse from the wave-shaper 22 in the speed error detection section 20 by a predetermined frequency-dividing ratio (1/m), a subtracter 32 for subtracting a reference phase Pref from a phase of the CFG pulse frequency-divided by the frequency-divider 31 and outputting the resultant value as the recording phase error signal, and a phase correction filter 33 for inputting the recording phase error signal from the subtracter 32 and outputting the inputted signal as the phase control data to perform a phase correction of the video tape in the recording mode.
The playback phase error detector 30B includes a wave-shaper 34 for inputting through the control head 5 the CTL signal, which is generated from the CTL signal generation section 70 in the recording mode and then recorded on the control track of the video tape, and wave-shaping the inputted CTL signal into the CTL pulse in the form of a square wave, a converter 35 for converting a period of the CTL pulse from the wave-shaper 34 into a time value, a subtracter 36 for subtracting the reference phase signal Pref from an output signal of the converter 35 and outputting the resultant difference as the playback phase error signal, and a phase correction filter 37 for inputting the playback phase error signal from the subtracter 36 and outputting the inputted signal as the phase control data to perform a phase correction of the video tape in the playback mode.
The speed/phase control section 40 includes an adder 41 for adding the speed control data from the multiplier 26 in the speed error detection section 20 to the phase control data through the first and second switching sections 50 and 60 from the phase error detection section 30, a speed/phase correction filter 42 for outputting speed/phase correction data in accordance with an output signal from the adder 41, a digital/analog converter 43 for converting the speed/phase correction data from the speed/phase correction filter 42 into an analog signal, and a motor driver 44 for driving the capstan motor 9 in response to the analog signal from the digital/analog converter 43 to vary an amount of the rotation thereof.
The operation of the conventional speed/phase control circuit with the above-mentioned construction will hereinafter be described with reference to FIGS. 4A to 4G.
In the speed error detection section 20, the CFG detector 21, during the travelling (recording and playback) of the video tape, detects the rotation of the capstan motor 9 from N/S magnetic poles of a magnet mounted in the capstan motor 9 and then outputs successively the CFG signals as shown in FIG. 4A.
The detected CFG signals from the CFG detector 21 are applied to the wave-shaper 22, which wave-shapes the CFG signals in the form of square wave to output the CFG pulses as shown in FIG. 4B.
The period measurer 23 measures the period of each of the CFG pulses from the wave-shaper 22. Namely, the period measurer 23 measures the period of the first inputted one ("1" in FIG. 4B) of the CFG pulses as shown in FIG. 4B and then the period of the second inputted one ("2" in FIG. 4B) of the CFG pulses. In this manner, the period measurer 23 measures sequentially the period of each of the CFG pulses from the wave-shaper 22.
The period value obtained by the period measurer 23 is applied to the converter 24, which converts the period value into the time value as shown in FIG. 4C. Namely, as shown in FIG. 4C, when the period of the CFG pulse obtained by the period measurer 23 is long, the time value of the converter 24 is large. In terms of hardware, the period measurer 23 may be a counter. Provided that a clock period of the counter is Tck, the output of the converter 24 can be expressed as 1/Tck.
The subtracter 25 subtracts the reference speed signal Sref from the speedometer from the output signal n.sub.CFG from the converter 24 and outputs the resultant difference as the speed error signal as shown in FIG. 4C. The speed error signal from the subtracter 25 is multiplied by the predetermined integral number KO from the speedometer by the multiplier 26, which outputs the resultant value as the speed control data.
On the other hand, the CTL signal generation section 70 generates the CTL signal in response to the recording mode signal S1. The CTL signal from the CTL signal generation section 70 is recorded on the control track of the video tape by the recording/playback control head 5.
In the phase error detection section 30, in the recording mode, the frequency-divider 31 frequency-divides the CFG pulse from the wave-shaper 22 in the speed error detection section 20 by the predetermined frequency-dividing ratio (1/m) as shown in FIG. 4D. The subtracter 32 subtracts the reference phase signal Pref as shown in FIG. 4E, from the CFG pulse frequency-divided by the frequency-divider 31 and outputs the resultant value as the recording phase error signal as shown in FIG. 4F. The recording phase error signal from the subtracter 32 is filtered by the phase correction filter 33 and then outputted as the phase control data.
In the playback mode, the recording/playback control head 5 reproduces the CTL signal which is generated from the CTL signal generation section 70 in the recording mode and then recorded on the control track of the video tape, as shown in FIG. 4G, and then applies the reproduced CTL signal to the wave-shaper 34. The wave-shaper 34 wave-shapes the inputted CTL signal into the CTL pulse in the form of square wave as shown in FIG. 4H and then applies the CTL pulse to the converter 35, which converts the period of the CTL pulse into the time value. The subtracter 36 subtracts the reference phase signal Pref from the output signal from the converter 35 and outputs the resultant difference as the playback phase error signal. The playback phase error signal from the subtracter 36 is filtered by the phase correction filter 37 and then outputted as the phase control data.
The speed/phase control section 40 is adapted to input the speed control data from the speed error detection section 20 and the phase control data from the phase error detection section 30 and control the speed and phase of the capstan motor 9 in accordance with the inputted data.
In the playback mode, the first switching section 50 selects the output of the playback phase error detector 30B in the phase error detection section 30 in response to the recording mode signal S1. The second switching section 60 applies the output of the playback phase error detector 30B selected by the first switching section 50 to the speed/phase control section 40 because the present mode is not the FF/REW mode. As a result, in the speed/phase control section 40, the adder 41 adds the speed control data from the multiplier in the speed error detection section 20 to the phase control data through the first and second switching sections 50 and 60 from the playback phase error detector 30B in the phase error detection section 30. The output of the adder 41 is filtered by the speed/phase correction filter 42 and then outputted as the speed/phase correction data for the capstan motor 9. The digital/analog converter 43 converts the speed/phase correction data from the speed/phase correction filter 42 into the analog signal and then applies the analog signal to the motor driver 44, which drives the capstan motor 9 in response to the analog signal from the digital/analog converter 43 to vary the amount of the rotation thereof.
In the recording mode, the first switching section 50 selects the output of the recording phase error detector 30A in the phase error detection section 30 in response to the recording mode signal S1. The second switching section 60 applies the output of the playback phase error detector 30A selected by the first switching section 50 to the speed/phase control section 40. Therefore, the speed/phase control section 40 controls the speed and phase of the capstan motor 9 in accordance with the inputted data in a similar manner to that in the playback mode.
On the other hand, in the FF/REW modes, the second switching section 60 is not connected to the output of the phase error detection section 30, but to the ground. For this reason, the adder 41 in the speed/phase control section 40 is applied with only the speed control data from the speed error detection section 20 with no application of the phase control data (i.e., .apprxeq.0). As a result, in the FF/REW modes, only the speed of the capstan motor 9 is controlled by the speed/phase control section 40.
However, the conventional VCR speed/phase control circuit has a disadvantage in that the phase error data cannot accurately be detected in a blank interval of the CTL pulse. The inaccurate detection of the phase error data results in an unstable travelling speed of the video tape and the an error occurs in determining the remaining amount of the video tape. Also, it is difficult to discriminate the type of the multiple speed-recorded video tape in the FF/REW modes.
FIG. 5 is a view illustrating a positional relationship of the tape travelling system in the case where a different video signal is to be recorded on the video tape under the condition that video, audio and control signals have previously been recorded on corresponding tracks of the video tape.
As the video tape 2 winds the direction as shown by the arrow in FIG. 5, the full erase head 3, the drum 4 and the recording/playback control head 5 (or an audio recording/playback head & CTL head assembly) are positioned as shown in FIG. 5.
As shown in FIG. 5, the video tape 2 is divided into four intervals T1, T2, T3 and T4. In the interval T1 of the video tape 2, the existing video/Hi-fi, audio and CTL signals are recorded on the corresponding tracks. In the interval T2, the video, audio and CTL signals are all erased by the full erase head 3. In the interval T3, only a new video signal is recorded on the video track by the drum 4. In the interval T4, new CTL and audio signals are recorded on the corresponding tracks by the CTL head 5 and an audio head (not shown), together with the new video signal.
Assume that the video signal is recorded, the travelling of the video tape is stopped, the rewinding operation of the video tape is performed and the playback operation is then performed. In this case, in the intervals T2 and T3, the CTL pulse is not recorded on the control track of the video tape. Namely, these intervals T2 and T3 are the blank intervals of the CTL pulse. The blank intervals of the CTL pulse are present whenever the record-stop-rewind-playback operations are performed.
For this reason, the blank intervals (T2 and T3) of the CTL pulse appear when the recorded signal is played back from the video tape 2. As a result, since the CTL pulse cannot be detected in the blank intervals, the operation of the playback phase error detector 30B in the phase error detection section 30 in FIG. 3 becomes unstable. The unstable operation of the playback phase error detector 30B causes the capstan motor 9 to be driven at a varying phase, resulting in an unstable travelling speed V of the video tape.
As mentioned above, the unstable operation of the VCR servo system adversely affects the ability to determine the remaining amount of the video tape.
When winding the video tape, the type of which is not discriminated, and in the direct FF/REW modes directly with no playback operation, the time required in discriminating the type of the multiple speed-record (for example, treble speed) video tape is three times as much as that in the standard speed-recorded video tape because the servo system is maintained at the playback condition according to the principle of the deck mechanism. Namely, the time required in the discrimination of the type of the video tape has an affect on the FF/REW time of the video tape.